When your ATV loses some of the fire in its belly, and you can’t quite put your finger on it. It may be time to check the valve lash. Your ATV engine is designed with a ton of power, and it’s still there, waiting to be tapped.
The top seven symptoms of poor ATV valve adjustment include:
1 ATV won’t start
2 Engine noisy
3 Engine misfires
4 Bike lacks power
5 Engine runs rough
6 Erratic idle
7 Hard on gas
By the end of this post, you’ll understand and recognize common valve lash symptoms, know which condition your engine suffers from, and how to fix it.
I’ve covered the process of adjusting the valves previously, which you may find helpful – ATV valve adjusting in 6 steps
This guide covers the more common rocker-style valve train. If your ATV is fitted with an OHC (Over Head Cam), the procedure is a little different.
What Is Valve Lash
Valve lash or clearance is a factory-specified gap that must be maintained between rockers (or cams) and valve tips. As an engine wears, the valve clearance usually gets larger and so needs to be adjusted. Unfortunately, on some bikes gaining access to the valves is a bit of a chore and is regularly ignored until it can’t be.
All single-cylinder engines will have at least two valves, one inlet, and one exhaust. However, most bikes will have four, two of each.
The valves, as you know, are activated by the rockers (cams in OHC engines), which are driven by the camshaft. The camshaft is driven by the crankshaft, and the whole process is sequenced. The timing of this sequence is mission-critical to the optimum performance of your power plant.
ATV 4 Stroke Engine
Only four-stroke engines have valves, and valves allow for more accurate, predictable, smoother, and cleaner engine performance. A four-stroke engine is just easier to live with, but because it has more internal moving parts than a two-stroke engine, it needs more maintenance.
The 4 strokes are:
1 Intake stroke – Intake valve(s) opens as the piston moves down the cylinder and closes again as the piston bottoms out.
2 Compression stroke – Intake valve(s) now closed as the piston moves up the cylinder, compressing the air/fuel mix into the combustion chamber at the top of the cylinder.
3 Power stroke – The spark plug fires and ignites the compressed mix causing the piston to move down the cylinder under power.
4 Exhaust stroke – Exhaust valve(s) open as the piston moves up the cylinder, pushing the spent gases out. The valve is fully closed as the piston reaches the top of the cylinder, and the cycle starts over.
I’ve covered valve lash adjusting previously, and you can check it out here – ATV valve adjusting in 6 steps
ATV Won’t Start
An ATV may not start for many reasons. Incorrect valve lash will certainly be on the list. Hot start problems are often caused by valve lash issues. As the engine reaches operating temperature, lash tolerances change. If the valves are too tight, they may remain open, reducing cylinder compression.
If you are concerned about compression and need to check it, try this post – How much compression should an ATV have?
An engine without compression won’t start. If the valve lash is excessive, your valves won’t open, and the engine won’t receive fuel and so won’t start or will be hard to start.
Engine Noisy
A noisy engine is a useful clue. The tell-tale ticking noise of an idling engine could very well be a maladjusted valve. Loose rockers, as you can guess, will make a racket if the gap between the rocker and valve tip is excessive. And you already know that excessive lash will cause poor running or no start.
Excessive lash, if neglected, can damage the valves and rockers as they hammer together. Checking the lash is the best test, but I’ll often use a stethoscope to confirm. Especially useful if access to the valve cover is labor-intensive.
Engine Misfire
A misfiring engine could be caused by miss adjusted valve lash. Excessive lash will cause the engine to run lean (not enough gas), and that will cause the plug to misfire. Running the engine with tight valves can also cause the plug to misfire.
If the valves are not closing fully, the engine mix is lean, and that will, as you know, cause a misfire.
Lacks Power
If your engine lacks power, maladjusted valves could be the cause. Incorrect valve lash causes the valves to open for more or less time than needed. Excessive valve lash means the valves open late and for less time than needed. This causes a lack of gas which in turn causes a lack of engine performance.
Tight valves can cause the valves to open early and for too long. This causes (depending on how tight they are) lean running, hot running, misfiring, and a general lack of performance.
Runs Rough
A rough-running engine is irritating but worse than that. If it’s caused by maladjusted valves, you could cause lasting damage to the motor. If valves are tight, it may cause the engine to run lean. A common symptom is a popping noise from the engine.
If a popping noise sounds familiar, you can read more about that symptom here – Why does my ATV make a popping noise?
Overheating engines, burnt valves, hammered valve tips, and blown head gaskets are all real risks that will cost a packet to fix.
Check out the rough running category for general ATV rough running symptoms.
Erratic Idle
Valve clearance will affect idling big time. If valves are tight, you might find the engine starts but then dies as it warms up. Or the idling is even rougher when warm. Whatever the symptoms, your engine valve lash should be checked once a year. If I owned a used bike, I’d assume it hasn’t been done recently, as it’s commonly overlooked.
Adjusting the valve lash is a bit of a chore on some bikes, but it’s worth it. You will notice the difference right away. The engine will start quicker and quieter, and the throttle response will be sharp and more powerful.
I wrote a post previously about an erratic idle, which you may find helpful – ATV idles up and down
Hard On Gas
Whenever performance suffers, so does gas mileage. It simply takes more gas to do the same work. So anytime you notice a gas mileage drop, it’s a big clue that your ATV has a problem.
The problem could be something simple like a dirty spark plug or more serious altogether. Either way, early diagnosis is always best.
Anyhow, no need to wonder if valve adjustment is your problem; you can check valve clearance and adjust valves in this post – ATV valve adjusting in 6 steps
You may find the following posts helpful:
I’ve written a ton of ATV troubleshooting posts. Hopefully, you won’t need them, but if you do, we have you covered.
- About the Author
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John Cunningham is a technical writer here at ATVfixed.com. He’s a Red Seal Qualified Service Technician with over twenty-five years experience. He’s worked on all types of mechanical equipment, from cars and trucks to ATVs and Dirt bikes.